Uria lomvia

Calls

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Adult Thick-billed Murre, breeding plumage, St. Paul, Alaska, July

Robust and hardy, the Thick-billed Murre is one of the most numerous marine birds in the Northern Hemisphere. In summer, it is found in continental-shelf waters of the Arctic Ocean and adjacent seas, shifting southward in winter. In terms of biomass, it is the dominant seabird over large parts of the Arctic Ocean in summer. It nests on precipitous sea cliffs in very dense colonies than can number over 1 million breeding birds. In the Pacific sector of the Arctic, breeding colonies are frequently shared with the congeneric Common Murre (Uria aalge), but in the Atlantic there is less overlap between the 2 species. The Thick-billed Murre is one of the deepest underwater divers of all birds, regularly descending to depths of more than 100 m, and occasionally below 200 m, and can remain submerged for more than 3 minutes. A generalist feeder, it takes most kinds of marine invertebrates and fish, from pteropod mollusks weighing less than 0.5 g up to fish weighing more than 50 g. Because of high wing-loading, its load-carrying capacity is poor and consequently chicks are provisioned only small amounts at each visit: usually a single fish. This limitation, combined with an open nesting site that requires chick-guarding by 1 parent throughout the rearing period and long commuting distances to feeding areas at many colonies, probably has been the main factor selecting for early departure of nestlings. Chicks leave the colony when only partially grown, at about one-quarter adult mass, and before flight feathers have begun to develop. At departure, they use the covert feathers, extended as miniature wings, to flutter and glide from the breeding site to the water. Growth is completed at sea, while being fed by the male parent alone. This unusual rearing strategy is otherwise seen only in the Common Murre and Razorbill (Alca torda).